1. Field of the Invention
The invention relates to an additive for the hydroconversion of heavy hydrocarbon oils. More particularly, the present invention is concerned with an additive which is useful for efficiently hydrocracking heavy hydrocarbon oils into lighter and more valuable oil products while suppressing the production of undesirable by-products. The present invention also relates to a method for the hydroconversion of heavy hydrocarbon oils by the use of the above-mentioned additive.
In the present specification, the terminology "additive" defines a hydrocarbon oil slurry containing a catalyst for the hydroconversion of heavy hydrocarbon oils, or precursors thereof.
2. Discussion of the Prior Art
The hydroconversion of heavy hydrocarbon oils defines a conversion of heavy oils having high boiling points, such as atmospheric or vacuum residual oils, into lighter hydrocarbon oils having lower boiling points, such as naphtha, kerosene, gas oil and vacuum gas oil. The hydroconversion is accomplished by heating the heavy hydrocarbon oils at a high temperature under a high hydrogen pressure. The hydroconversion also includes removal of so-called heteroatoms present in the feedstock heavy oil, such as sulfur, nitrogen, vanadium and nickel, which results in an upgrade of the properties of the produced lighter hydrocarbon oils by the hydrogenation thereof.
In methods for the hydroconversion, there is known a method in which a catalyst is suspended in feedstock heavy oils(hereinafter referred to as "catalytic slurry method"). It is generally acknowledged that this method is effective and useful because, according to this method, the hydroconversion can be effectively carried out even under severe conditions using as the feedstock, a heavy oil containing high concentrations of asphaltene, carbon residue, metallic constituents and ash, for example, vacuum residual oils, while preventing precipitation and deposition of carbonaceous solid substances formed by side reactions such as polymerization and condensation. The catalytic slurry method is advantageous in that the catalyst used is not deteriorated and the reactor is not plugged as opposed to the hydroconversion method using a catalyst in a fixed bed or fluidized bed.
Heretofore, various catalytic slurry methods have been proposed as follows.
U.S. Pat. Nos. 4,134,825, 4,285,804 and 4,548,700 disclose a hydroconversion method in which the hydroconversion of heavy hydrocarbon oils are effected in a system in which a transition metal compound (a catalyst precursor) or a catalyst obtained by decomposing the transition metal compound, is suspended in the heavy hydrocarbon oils so that hydrogenating ability of the transition metal compound may be exerted.
U.S. Pat. Nos. 4,299,685, 4,169,038, 4,406,772 disclose a method in which a solid substance, such as coal ash powder and by-product coke, is suspended in a heavy hydrocarbon oil and the hydroconversion of the oil is carried out mainly by utilizing hydrogen pressure.
Further, there are known hydroconversion methods in which, a solid substance having, supported thereon or impregnated therein, a metal compound which is similar to the state of the catalyst used in a fixed-bed method or a fluidized method, is suspended in a heavy hydrocarbon oil. For example, U.S. Pat. Nos. 4,214,977, 4,495,306 and 4,557,822 disclose a method in which, a metal salt-impregnated coal powder, is suspended in a heavy hydrocarbon oil, and U.S. Pat. No. 4,357,229 discloses a method in which a metal powder having a decomposition product of an oil-soluble metal compound supported thereon
Moreover, there are known hydroconversion methods in which a customarily employed metal compound having hydrogenating activity and a powder or granule of a solid substance are separately suspended in a heavy hydrocarbon oil. For example, U.S. Pat. No. 4,376,037 discloses a method in which a granular porous refractory inorganic substance is suspended in a heavy oil together with a metal compound; U.S. Pat. No. 4,431,520 discloses a method in which by-product metal-containing soot particles (cenospheres) are suspended in a heavy oil together with a metal compound; and Japanese Patent Application Laid-Open Specification No. 60-120791 discloses a method in which an ultra fine particulate substance is suspended in a heavy oil together with a metal compound
In recent years, with respect to petroleum resources, the supply of heavy oils has been increasing However, with respect to the petroleum products, the demand for lighter oils has also been increasing. Therefore, the uneven balance between the supply of heavy oils and the demand for lighter oils has become a social problem. In order to solve the problem, it is earnestly desired in the art to develop a technically and practically advantageous method for effectively converting heavy crude oils into the more valuable lighter oils. For example, a method for continuously converting vacuum residual oils having boiling points of 538.degree. C. or more into lighter oils having boiling points lower than 538.degree. C. at a conversion level as high as at least 80% by weight, preferably at least 85% by weight, and yet more preferably at least 90% by weight.
To obtain such highly efficient hydroconversion, it has been necessary to suppress the formation of coke- or asphaltene-like polycondensation by-products having been formed by side reactions which inevitably occur in the reaction apparatus, particularly in the reaction zone of the reaction apparatus and to prevent precipitation and deposition of such polycondensation by-product (i.e., scaling or coking) in the reaction apparatus. Further, it has been required that the yield of lighter oils be increased while suppressing excessive gas generation. Moreover, it has been required that the hydrogenation of the hydroconversion products (lighter oils) be effectively performed in order to remove heteroatoms such as sulfur atoms, nitrogen atoms, etc. Further, in the catalytic slurry method in which hydroconversion is conducted in a continuous manner, in order for the method to be rendered simple and easy, at least part of the catalyst is discarded after use. Therefore, it is desirable that the catalyst to be used be effective, even when it is used in a small amount. Accordingly, a catalyst which is expensive or troublesome to produce should not be used.
In addition, there is the problem of handling the residue after recovering low boiling point distillates from the hydroconversion products. In general, hydroconversion is conducted in a continuous manner. In this case, it is desirable that the residue is capable of being used as fuel oils without the necessities of removing or recovering the catalyst therefrom. Thus the hydroconversion process becomes simpler and the operation cost is lowered. However, when the hydroconversion of a raw material heavy oil is conducted at high conversion such that 80% by weight or more of the feedstock heavy oil is converted into lighter oils, residue is formed which has a boiling point higher than 538.degree. C. and which contain the catalyst as well as the polycondensation by-products formed in the reaction zone of the reaction apparatus at concentrations which are at least 5 times, sometimes at least 10 times greater than the concentrations before the hydroconversion reaction. In order for the residual oils formed in the hydroconversion of heavy oils to be fluid and combustible, it is requisite that the catalyst and polycondensation by-products be sufficiently minute and the total content thereof in the residual oils be as low as 40% by weight or less. Further, in order to decrease the amount of ash which is formed when the residual oils are burned, refractory inorganic substances which are conventionally used as support for a catalyst should not be used, or even if used, the amount thereof should be decreased as much as possible.
However, up to the present time the above-mentioned conventional catalytic slurry methods have not been found to be satisfactory.